Motor vehicle generators are usually designed in such a way that they satisfy as well as possible the power demands within a certain voltage level, for instance, a voltage level of 14 Volt and a rotary speed population. To do this, the generator is regulated, using a generator regulator, in such a way that the output voltage, that is, in principle, a function of the rotary speed, is held constant over the entire possible rotary speed range. In generators that are designed for a first operating voltage of 14 Volt, for instance, at a second operating voltage that is higher than the first operating voltage, greater power could be attained at a clearly improved efficiency. The so-called turn-on rotary speed, that is the rotary speed beginning at which the generator is able to give off power, also increases, however, with increasing operating voltage.
German Patent Application No. DE OS 198 59 036 describes a vehicle electrical system for a motor vehicle, in which a generator is used whose output voltage is modifiable, and is freely selectable, for example, to lie between 14 Volt and 42 Volt. In this context, the generator is connected via an in-phase regulator to the vehicle electrical system, which requires a constant voltage of 14 Volts, for instance. The battery, having the nominal voltage of 12 Volts, which is to be charged with approximately 14 Volts, also belongs to this vehicle electrical system. Users which can also be operated using a higher voltage than 14 Volts, for instance, an electrical heating system, an engine radiator fan, etc., are operated at a variable voltage of 14 Volts to 42 Volts.
A generator, which is usually designed as a three-phase generator, is generally able to generate overvoltages when loads are suddenly switched off. That is why it is customary to provide a generator, that is located in the vehicle electrical system, with a so-called load dump protector which, for example, includes Zener diodes, and reduces the effect of overvoltages, and prevents the overvoltages from leading to damage. An example of a generator having Zener diodes is described in German Patent Application No. DE-OS 101 18 846.
In a multi-voltage onboard electrical system in which the generator is operated at free voltage, if users are suddenly switched off, this can lead to a large voltage pulse which, for example, at an operation at 42 Volts, could lead to the destruction of the Zener diodes. Zener diodes in current load dumping protective circuits are designed for power in the forward direction, and are therefore only able to take on a part of the load dumping current without being damaged themselves. In a complete load breaking at a generator operation in the higher voltage range, damage to the Zener diodes would be probable.
Therefore, in German Patent Application No. DE-OS 101 18 846, in connection with a multi-voltage onboard electrical system, a special interconnection of Zener diodes is described in which, in the load dumping case, the Zener diodes are operated at least partially at their breakdown, that is, in backward operation. In this instance, the limitation of the voltage is determined mainly by the Zener voltage of those diodes that are being operated in backward operation.
An alternative to, or refinement of the conventional design described above, could be to obtain larger diode chip areas by using larger diodes or by the parallel connecting of several diodes, or by using a load dump-protective unit that includes high blocking diodes. Voltage-sensitive users could be switched off by a load management if a load dump is threatening to occur. Similarly, as of the attainment of a voltage threshold, voltage-sensitive users could be selectively switched in.